Conventionally, a holding magnet type electromagnetic actuator has been well known for applying mechanical force to a valve rod, piston or the like.
Referring to FIG. 1, there is shown this holding magnet type electromagnetic actuator which comprises a permanent magnet 2, and a pair of cores 1a and 1b which are respectively wound around with solenoid coils 3a and 3b. The permanent magnet 2, cores 1a,1b and the solenoid coils 3a and 3b are so arranged as to form a magnetic circuit wherein magnetomotive forces of the electromagnet and the permanent magnet are arranged in series; that is, the magnetomotive force of the solenoid coils 3a and 3b is generated in the counter direction of the coercive force of the permanent magnet 2 when an electric current is flowed through the solenoid coils 3a and 3b. According to this arrangement, a contact element 4 can be reversibly held in either state shown in FIG. 1(a) or FIG. 1(b).
When an electric current is so flowed through the solenoid coil 3a under the condition shown in FIG. 1(a) as to generate the counter magnetomotive force against the coercive force of the permanent magnet 2, the contact element 4 will be attracted to the another core 1b which is connected to the permanent magnet 2 and consists of magnetic material having great coercive force as shown in FIG. 1(b). On the other hand, when an electric current is so flowed through the solenoid coil 3b in the state of FIG. 1(b) as to generate the counter magnetomotive force against the coercive force of the permanent magnet 2, the contact element 4 is returned to the initial state shown in FIG. 1(a).
Although this holding type electromagnet has a self-holding capability for the contact element 4 when an electric current is not flowed, it has essentially the following demerits.
(1) This type actuator requires two sets of solenoid coils 3a and 3b for actuating and returning operations so that the structure will be complicated and the size will be enlarged.
(2) An electric current is so flowed through the solenoid coil 3a or 3b as to generate the magnetomotive force in the counter direction of the coercive force of the permanent magnet 2 in order to reduce the coercive force so that the required ampere turn will be increased. Accordingly, an electric power of at least 10 W is required to generate the propulsive force of 0.2 kg and stroke of 2 mm.
(3) This type actuator requires three electric wires to control the actuation.